1. Field of the Invention
This invention relates to brake calipers for use with disc brakes. In particular, the invention relates to pistons used in brake calipers.
2. Discussion of Related Art
In typical disc brake assemblies, a rotor is mounted on a wheel hub for rotation. One or more pairs of brake pads, generally designed as friction material carried on a backing plate, are supported on either side of the rotor by a caliper or an anchor bracket. Calipers are designed to apply the braking force by moving a piston relative to the rotor to thereby move the brake pad into and out of contact with the rotor. The pistons are supported in a bore and abut the backing plate of the brake pad to move the brake pad toward the rotor upon application of a braking force. The caliper can be hydraulically actuated, for example, to selectively press the brake pads against the surface of the rotor to stop the rotation of the rotor and hence the wheel hub by forcing hydraulic brake fluid into the piston bore.
The piston bore has an annular seal retained in the bore that seals the brake fluid in the bore and causes the piston to retract into the bore after the braking force is removed. The seal is compressed by the piston. So, when the piston is forced out of the bore toward the rotor by the brake fluid, the seal is pulled with the piston. When the braking force is removed, the seal tends to roll back into the bore pulling the piston with it.
Pistons are often made of lightweight material. As the braking force applied by the piston to the brake pad can be large, piston caps are often used on the piston surface that abuts the backing plate of the brake pad to strengthen the piston and increase its durability. The caps are generally a hard material, such as steel. However, a great deal of heat is transferred through the cap. One way of reducing the heat transfer is to use a stainless steel cap or titanium cap.
Upon application of the brake pads to the braking surface of the rotor, friction generated between the brake pad and the braking surface slows rotation of the rotor. This generates heat that is transferred from the brake pad through the backing plate to the piston and caliper assembly. Pistons that have stainless steel caps can transfer as much as 4-5 watts/minute/degrees Kelvin (W/min/xc2x0 K.) to the piston and caliper. The amount of heat generated can vary, but is often extreme in high performance applications, such as in racing vehicles, heavy duty vehicles, and other extreme situations. Thus, even with the use of a stainless steel cap, too much heat is transferred to the caliper.
The high heat generated during braking has several adverse effects to the caliper assembly. First, the piston seal typically only has an operating range up to 300xc2x0 F. When the heat generated during braking exceeds that temperature, the seal can become degraded and crack or lose its elasticity. When the seal does not function properly, brake fluid can leak from the bore, which causes the brake to malfunction. The loss of elasticity prevents the seal from pulling the piston back into the bore, which causes the brake pad to continue to rub on the rotor surface. This condition is called xe2x80x9cdragxe2x80x9d and will slow the vehicle, create excessive wear on the rotors and pads, generate additional heat, and adversely affect fuel economy.
Second, heat affects the brake fluid in the caliper assembly. If heat is transferred to the piston and hence the brake fluid surrounding the piston, the brake fluid can be heated to the point of boiling. Once the brake fluid boils, the fluid is no longer incompressible. As a result, braking is more difficult or can not be accomplished at all. This effect causes a condition often termed xe2x80x9clong pedalxe2x80x9d in which a longer pedal stroke is required to cause the piston to actuate the brake pad.
Third, increased temperature affects the caliper itself It is preferred that the caliper be as stiff as possible to securely and uniformly hold the pistons in position for braking. Many calipers are formed of aluminum for its lightweight strength. However, aluminum functions best at temperatures below 350xc2x0 F., with the stiffness dropping considerably at higher temperatures. These higher temperatures will cause more deflection of the caliper, which in turn leads to increased brake fluid disbursement. As a result, the driver will experience a long pedal.
Thus, there is a need for an insulated assembly that reduces heat transfer from the brake pad to the caliper assembly.
An aspect of embodiments of the invention is to provide a caliper assembly with insulating characteristics.
Another aspect of embodiments of the invention is to provide a caliper assembly with a piston having an insulating cap.
A further aspect of embodiments of the invention is to provide a caliper assembly having a piston that includes a high performance polymer insulator, such as a polyimide resin.
These and other aspects of the invention can be realized by embodiments of the invention in which a piston is provided for use with a brake caliper, the piston comprising a piston body, a cap piece secured to an end of the piston body, and an insulator having low thermal conductivity disposed between the cap piece and the piston body.
The invention is also directed to a brake caliper comprising a caliper body configured to surround an edge of a rotor, wherein the caliper body includes at least one bore, a piston slidably retained in the bore and having an end protruding from the bore, and a composite cap secured to the end of the piston. The cap includes a cap piece having an attachment element that couples the cap piece to the piston and an insulator disposed between the cap piece and the piston.
The invention is additionally directed to a disc brake assembly comprising a rotor including a disc and an attachment portion that is configured for attachment to a wheel hub, at least one pail of brake pads mounted adjacent to the caliper on either side of the rotor for selective engagement with the rotor, and a brake caliper mounted adjacent to the rotor so as to surround an edge of the rotor. The brake caliper includes at least one piston slidably mounted adjacent the brake pads to selectively actuate movement of the brake pads toward the rotor. Each piston has a piston cap having a metallic cap piece and an thermal insulator disposed between the cap piece and the piston. The disc brake can be combined with a vehicle.